1. Field of the Invention
The present invention relates to a novel electrochromic device.
2. Description of the Prior Art
An electrochromic device (to be referred to as an ECD hereinafter) comprises a pair of electrode layers at least one of which is transparent and an electrochromic material layer sandwiched therebetween. The ECD colors when a voltage having a magnitude as substantially obtained by a dry battery is applied between the pair of electrode layers. However, when a voltage having an opposing polarity is applied between the electrode layers, the ECD is bleached to return the original transparent state. For this reason, extensive studies have been made to utilize ECDs in a display unit (especially, a numerical display device using a 7-segment display element of an " " shape), a transmitted or reflected light amount control device, and others.
In order to color an electrochromic material such as WO.sub.3, electrons (e.sup.-) and cations (X.sup.+) are simultaneously injected in the electrochromic material. It is assumed that the general reaction upon coloring/bleaching of the material is written as follows: ##STR1## Protons (H.sup.+) are used as the cations (X.sup.+) since the proton has a small ion radius and high mobility. The cations need not always be in the state of cations. More specifically, it is essential to generate cations when a voltage is applied across the electrochromic device. For protons, water is used as a cation source. Water is decomposed upon application of a voltage thereto in accordance with the following general reaction: EQU H.sub.2 O.fwdarw.H.sup.+ +OH.sup.-
Only a small amount of water is required, so that moisture permeated from the outer atmosphere into the WO.sub.3 layer is usually sufficient.
However, even if the WO.sup.3 layer is sandwiched between the pair of electrode layers and a reverse voltage is applied between these electrode layers, bleaching cannot be easily performed. This is because electrons (e.sup.-) flow in from the electrode corresponding to the cathode even if the reverse voltage is applied across the electrodes so as to bleach the electrochromic material, thereby coloring the electrochromic material due to the presence of cations (H.sup.+) in accordance with the following reaction: EQU WO.sub.3 +ne.sup.- +nH.sup.+ .fwdarw.HnWO.sub.3
In order to solve this problem, an ECD is proposed by S. K. Deb et al. in Japanese Patent Publication No. 46098/1977 corresponding to U.S. Pat. No. 3,521,941 (Re. 28199), wherein an insulating layer of SiO.sub.2, MgF.sub.2 or the like is formed between the WO.sub.3 layer and one of the electrode layers. Electrons cannot be moved through this insulating layer, but OH.sup.- ions can be freely moved therethrough. These OH.sup.- ions carry an electric charge. This insulating layer is called an ionic conductive layer throughout the specification of the present invention.
The following reaction occurs between the ionic conductive layer and the electrode: EQU OH.sup.- .fwdarw.(1/2)H.sub.2 O+(1/4)O.sub.2 .uparw.+e.sup.-
It is assumed that the electrons are emitted toward the anode.
In other words, when the ECD colors, the following reactions are assumed to occur: EQU (anode) WO.sub.3 +ne.sup.- +nH.sup.+ .fwdarw.HnWO.sub.3 EQU (cathode) n(OH.sup.-).fwdarw.(n/2)H.sub.2 O+(n/4)O.sub.2 .uparw.+ne.sup.-
However, when the ECD is bleached, the following reactions are assumed to occur: EQU (anode) HnWO.sub.3 .fwdarw.WO.sub.3 +ne.sup.- +nH.sup.+ EQU (cathode) nH.sub.2 O+ne.sup.- .fwdarw.nOH.sup.- +(n/2)H.sub.2 .uparw.
As is apparent from the above reactions, the ECD proposed by S. K. Deb et al. has disadvantages in that coloring does not occur when moisture is not immmediately supplied thereto and that the layers are peeled off due to the generation of O.sub.2 and H.sub.2 gases.
In order to overcome the drawbacks of the ECD proposed by S. K. Deb et al., Y. Takahashi et al. proposed all solid-state ECD having iridium hydroxide as a reversibly oxidizable layer between the ionic conductive layer and the electrode layer (see U.S. Pat. No. 4,350,414). It should be noted that the WO.sub.3 layer is not only a reversibly reducible layer but also a cathodically coloring electrochromic layer.
In the ECD proposed by Y. Takahashi et al., iridium hydroxide is assumed to be subjected to the following reaction during coloring of WO.sub.3 : ##STR2## Iridium hydroxide is assumed to be subjected to the following reaction during bleaching: ##STR3## Therefore, water is not consumed but regenerated in this ECD, and H.sub.2 and O.sub.2 gases are not generated. It should be noted that this ECD having a symmetrical structure need not use the ionic conductive layer when a memory function is not required.
However, the ECD of Y. Takahashi et al. was not completely bleached, i.e., transparent when a high-temperature durability test for a long period of time was performed. As a result, it was found that contrast between coloring and bleaching was degraded.